i will make my answer pertain to book seven, having read book six. i think it is possible that Voldemort could in fact breech hogwarts, after he has one of the Marauders (Peter Pettigrew, AKA Wormtail) on his side, and Wormtail helped create the marauders map, which means he knows of all the known secret passages into Hogwarts. this said, he may very well use the passage that goes through the honey dukes cellar or one of the others. but i dont like the idea of hogwarts being breeched at all.

"Mistletoe," said Luna dreamily, pointing at a large clump of white berries placed almost over Harry's head. He jumped out from under it. "Good thinking," said Luna very seriously. "It's often infested with nargles."

If it's wrong to think Tom Riddle is attractive, then i don't want to be right.

Well, to add to what has been said, we know that Voldemort made it into Hogwarts in HBP, or his cronies did, at least. He did it once, he can probably do it again. With Dumbledore gone, I doubt he would fear anyone else there enough to keep him away. I can't imagine why he would want to go there though unless it's to get an artifact or a hidden Horcrux. Maybe he thinks he can illicit terror into all the witches and wizards by showing them that their children are not safe even in Hogwarts. Maybe Voldemort will take over the school for no other reason than to draw Harry there because he knows Harry would go there to protect his friends.

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What did I care if numbers of nameless and faceless people and creatures were slaughtered in the vague future, if in the here and now you were alive, and well, and happy?

Albus Dumbledore

"I miss having you in my classes, Harry, you were never much of a Seer . . . but you were a wonderful Object . . . "